Modification of telechelic-type polymers



United States Patent 3,305,523 MODIFICATION OF TELECHELIC-TYPE POLYMERSCharles II. Burnside, Waco, Tex., assignor to North American Aviation,Inc. No Drawing. Filed Aug. 30, 1962, Ser. No. 221,654 4 Claims. (Cl.26046.5)

This invention concerns a novel solid propellant polymeric binder. Morespecifically, this invention pertains to a solid propellant binderhaving unexpectedly high elongation properties and high solid loadingability.

A typical composite solid propellant is comprised of solid particulatefuel such as minute particles of a metal and a solid particulateoxidizer which serves to oxidize the fuel particles. In order to securethe particles of oxidizer and fuel in an intimate mixture necessary foreffective combustion and practical use, a resinous matrix material orbinder is required. The binder material normally additionally serves asa fuel component of the solid propellant. This binder is normally aliquid resinous i material and when mixed with high percentages of thesolid particles of fuel and oxidizer, forms a viscous material which maybe cast in a mold and cured for the desired shaped propellant charge.Physical properties are desired in a solid propellant. These propertiesare normally in an interrelated combination in that the maximum diluteof all the parameters does not necessarily yield the best solidpropellant. One parameter depends upon the value of another in severalperformance evaluations. The solid propellant should be stable for along period of time and should not deteriorate chemically or physicallyduring storage. High density of the solid propellant permits the use ofa small chamber volume and therefore small chamber weight. Thepropellant should lend itself readily to production and have desirablefabrication properties such as adequate fluidity during casting or easyto control chemical processes such as curing and a minimal volume changeafter casting or molding. The mechanical properties and the combustioncharacteristics such as burning rate should be predictable anduneifected appreciably over a wide range of storage and operatingtemperatures. This means that the temperature sensitivity should be low.

Additionally the propellant should exhibit good mechanical properties,particularly its tensile, compressive and sheer strength and itsrnodulous of elasticity and elongation. Finally the propellant shouldhave high solids loading capability.

The previously set forth desirous properties in a solid propellant havebeen accomplished through the use of propellants which utilize acarboxy-terminated linear polybutadiene binder. The so-called Flexadynefamily of solid propellants was developed specifically to have superiormechanical properties in tension, compression, tear and creep and tomaintain a good balance of such properties over the temperature range 75to 170 F.

One of the components of this family of propellants that gives the groupthe outstanding properties is the propellant binder polymer utilized.The polymer binder used in these propellants is a carboxy-terminatedlinear polybutadiene. The excellent properties of the binder areattained when the starting initial molecules of butadiene have thehydrogen atoms at the double bonds in carefully ordered positions inspace to obtain a symmetrical cross-linking of the cured polymer.Additionally, the polymer has a properly controlled cistransstereoisomerism, when the polymerization is stopped at 100 molecules ofbutadiene per chain, when the number or nature of possible side chainsis concerned and when a carboxylic group is attached at either end ofeach polybutadiene molecule and nowhere else. The resulting symmetrical3,305,523 Patented Feb. 21, 1967 structure has good elastornericproperties and is achieved in the binder by controlling the chemistryand of the polymerization process itself. This type of polybutadienepolymer is referred to as a telechelic-type polymer and the method ofpreparation is well known in the art and is described in the Journal ofPolymer Science, vol. XLVI, Issue 148, pp. 535-539 (1960). An example ofthe carboxy-terminated polybutadiene binder used in this group ofpropellants is Butarez CTL manufactured by the Phillips PetroleumCompany. In addition to the carboxy-terminated linear polybutadienetelechelic-type polymers other possible terminal groups can successfullybe utilized in the present applications. Such other terminal functionalgroups would include hydroxy radicals, amine, mercapto and isocyanateradicals. It is understood that any telechelic polymer regardless of theterminal functional groups can be successfully utilized.

The present invention concerns the further improvement of mechanicalproperties of the telechelic polymer binders previously described.Particularly, the invention is directed to the improvement of themechanical properties of the binder material over a wide range oftemperature varying from less than -70 F. to excess of F. The greatlyimproved mechanical properties are accomplished in the present inventionby the extension of the telechelic polymer chains through increasing thechain length and molecular Weight by the addition of from 0.4 to 10parts by weight of difunctional materials, such as, for example,vinylcyclohexene dioxide. Particularly preferred is from 0.6 to 5 partsby weight of the difunctional additives. The difunctional additive thatis utilized depends upon the functional terminal group in the telechelicpolymer compound. The difunctional additive must be one that will reactwith the functional terminal group of the polymer in order for aneffective result to be obtained. As will hereinafter be set forth,various difunctional additives may be used depending upon the functionalterminal group of the telechelic polymer. Thus, as can be seen, thedifunctional additive becomes part of the polymer chains.

constancy of mechanical properties of a solid propellant or erratictemperature change is desirable so that brittleness at low temperatureand plastic flow at high temperatures do not limit the use of the rocketpropellant to a narrow temperature range. One of the many requirementsof a propellant charge is that it be plastic enough or have enoughelongation to change dimensions without cracking as required by therocket motor expansion under pressure. High elongation of the rocketpropellant is desirable to accommodate the differential expansionproperties between the motor chamber and the propellant. As a result, itis of great significance that the mechanical properties of a solidpropellant binder of a telechelic-type can be greatly improved throughthe addition of the difunctional compounds of this invention. Theelongation of the propellants utilizing the binders of this inventionhave increased 40 to 50 percent over a temperature range of 70 to 170 F.In some instances this represents a doubling of the percent elongationof the propellant.

The telechelic polymers used in the propellant binder as previouslydescribed are not only polybutadienes. The invention also pertains tothe improvement of the mechanical properties of telechelic polymerswhich may be of other skeletal structures such as any polymerizablemonomer containing an ethylenic linkage.

Non-limiting examples of the monomers that may be utilized in theinvention to produce the telechelic polymers include the vinyl halidessuch as iodoethene, bromoethene, fiuorethene, chloroethene; vinylidenehalides including vinylidene chloride, vinylidene iodide, and the like;vinyl esters of carboxylic acids having 1 to 10 carbon atoms includingvinyl acetate and the like; and N-substituted acrylamides andmethyacrylamide including N methylacrylamide, N-propyl methyacrylamide,N-hexyl aerylamide and the like; allyl and methya-llyl monomersincluding allylamine, N-methylallylamine, allyl bromide, allyl alcohol,allyl chloride, allyl cyanide, allyl fluoride, allyl glycidyl ether,allyl isodide, allyl mercaptan, allyl sulphide, and the like;isopropenyl monomers including isopropenyl bromide, isopropenylchloride, isopropenyl fluoride, and the like; vinyl compounds includingvinyl alcohol, vinyl bromide, vinyl fluoride, vinyl cyanide, vinylsulphide, vinyl tribromide, vinyl ether, vinyl napthalene, and the like;vinyl ethers including ethenylenyethene, propenyloxyethene, and thelike; acrylates and methacrylates including ethyl acrylate, methylacrylates, propyl methacrylates, hydroxy propyl methacrylate, hexylacrylate, and the like; glycidyl acrylates, .glycidyl methacrylates, andthe like; and styrene. Additionally, mixtures of the above monomerscould be used. For example, the copolymer of butadiene and styrene isapplicable.

The functional terminal groups that are present can be selected fromhydroxy, amino, mercapto and isocyanate radicals, in addition to anyother reactive functional groups. As previously described, the compoundswhich are added to the telechelic polymer to improve the mechanicalproperties of the propellants formed can be varied depending upon thefunctional terminal groups present in the telechelic polymer. When oneof the functional terminal groups of the telechelic polymer is a carboxyradical, the difunctional additives may include dialcohols such asdihydroxybenzene, dihydroxynapthaline, dihydroxybutane, and the like;diepoxides such as vinylcyclohexene dioxide, the Epon resins which arethe products of epichlorohydrin and bis-phenol-A, diamines such asethylene diamine, butylene diamine, hexamethylene diamine and the like;diamines such as hydrazine and the like; and lbis-chlorocarbonates suchas ethylene bis-chlorocarbonate and the like.

When the functional telechelic groups in the telechelic polymers arehydroxy radicals, the difunctional additives may be selected from thegroup consisting of diimines; dicarboxylic acid chlorides such assuccinyl chloride and the like; dicarboxylic acids such as maleic acidand the like; phosgene; bis-chlocarbonates and the like; anddiisocyanates which are formed by treating diamines such as toluene2,4-diamine, hexamethy aminediamine, and the like with phosgene.

When the functional telechelic groups in the telechelic polyers areamino groups, the difunctional additives may be selected from the groupconsisting of dicarboxylic acids; diamines; diepoxides; and dialdahydessuch as malonic aldehyde and succinic aldehyde.

When the telechelic polymers have terminal functional groups that aremercaptans, the difunctional additives utilized may be acid chlorides;bischlorocarbonates; and usually any compound containing two doublebonds (diolefins) capable of reacting with the mercaptan, such asethyleneglycol diacrylate, for example. It is believed the inventionwill be further understood from the following detailed examples.

When the telechelic polymers have terminal functional groups that areisocyanates, the terminal difunctional additives having difunctionalterminal hydroxy may be any compounds or amino groups or one of eachfunctional type.

Example 1 Into a two and one fourth gallon Baker-Perkins dispersionblade mixer was added 1000 grams of Butarez CTL which is acarboxy-terminated linear polybutadiene. Additionally added was 6 gramsof vinylcyclohexenedioxide. Water, at 165 F., was circulated through themixer heating jacket. The material was then blended for fifteen minutes.A 28-inch vacuum was then applied and mixing continued for an additionalten minutes to remove the air present. The contents of the mixer werethen poured into a large polyethylene bag. The bag was then sealed andthen placed in a metal bucket container, the bag serving two functions.(1) To prevent contact with metal surfaces and (2) to prevent contactwith air. The material was then heated for 168 hours at 170 F. A liquidmodified polymer of this invention was obtained upon cooling of thematerial at room temperature and the removal from the sealed bag. Theabove procedure was repeated utilizing 1000 grams of the Butarez CTL and10 grams of 1,3-bis[3 (2,3-epoxy-propoxy)propyl]tetra methyl disiloxane.A modified telechelic polymer of this .invention was obtained.

As is in the reaction of most polymeric-type materials, the temperatureand length of time of reaction varies considerably. The temperature ofreaction may vary from, for example, 15 0 F. to 225 F. with the time ofreaction varying according to the temperature utilized with the highertemperatures requiring less reaction time. Thus, reaction times can varyfrom 145 hours to hours. Basically more important than the temperatureand time of reaction ,is the assurance of conversion to the desiredproduct. During the reaction of the difunctional groups with thetelechelic polymers, samples of the mixture are taken from time to timeduring the reaction period. The samples are analyzed, for example, inthe case of the carboxy-terminated telechelic polymers to determine theephr value, that is the carboxyl equivalents. Once the carboxylequivalents of the mixtures stabilize, there is an indication that thereaction has gone to completion and all of the difunctional groups thatare 'going to react have reacted with the carboxy groups present. Atthis time the reaction is stopped. The same process could be taken when,for example, mercaptan groups are the terminal functional groups of thetelechelic polymer. Mercaptan equivalents would be determined during thecourse of the reaction until a stabilized condition is reached at whichtime the reaction has gone to completeion and can be stopped.

The invention has peculiar applicability to the field of solidpropellants wherein the polymers are used as binders to secure the solidparticulate matter utilized.

The solid material which may be dispersed throughout the polymer matrixis usually in finely divided form having a particle size ranging fromabout 1-500 microns or greater in diameter. When the composition isintended as a solid propellant grain, it is often desirable to employ acombination of two or more different particle size ranges. For example,solid propellants are prepared in which the finer material comprises afine particle size range of from 1 to about 75 microns and a coarserange of from about 75 to 500. However, particles of any size within therange of 1500 microns may be employed without regard to particle size.This gives desirable burning rates to the propellant. The particle sizeranges may be adjusted depending upon the particular binder-fueloxidizercombination employed and the specific impulse desired.

The solid substances with which the polymeric materials are loaded maybe inert pigments such as titanium dioxide, lead oxide, ferric oxide,carbon black, powdered metals and alloys, metal fluorides, asbestosfibers, etc.

When the solids are oxidizing agents, they can be compounds such asmetal perchlorates and metal nitrates. The metal perchlorates employedas oxidizing agents or oxygen carriers in the compositions are anhydrousand have the general formula M(ClO wherein M is NH or a metal and x isthe valence of M. Since the propellant composition is required towithstand high temperature storage, it is preferable that the meltingpoint and the decomposition temperature of the oxidizer be as high aspossible. The perchlorates of the Group I-A, Group- I-B, and Group II-Ametals are found to have the re-- quired high temperature stability andare employed in the preparation of propellant compositions by theprocess of this invention. Hence, the metal perchlorates used in thepreparation of the propellant compositions include lithium perchlorate,sodium perchlorate, potassium perchlorate, rubidium perchlorate, andcesium perchlorate which are the perchlorates of the metals of Group I-Aof the Periodic Table of Elements; silver perchlorate which is aperchlorate of the Group IB metal; and magnesium perchlorate, calciumperchlorate, strontium perchlorate, and barium perchlorate which are theperchlorates of the Group II-A metals. In addition to the metalperchlorates, the compound ammonium perchlorate finds extensive use inpropellant compositions. Examples of the nitrates of the Group IA, andI-B and II-B which are employed in preparing propellant compositions bythe process of this invention are compounds such as lithium nitrate,sodium nitrate, potassium nitrate, magnesium nitrate, calcium nitrate,barium nitrate, strontium nitrate, etc; Ammonium nitrate is also used.

The ratio of total solids-to-polymeric binder material in a propellantfalls in the range of from about 1:1 to about 9:1 with an optimum ratioof about 85:15.

Other substances which are employed in the preparation of propellants bythe process of this invention include minor amounts of burningcatalysts, well known in propellant compositions. These are composed ofone or a mixture of two or more metal oxide powders in amountssuificient to improve the burning rate of the composition. The amountsusually range from about 0.01 to about 3 weight percent, based on theweight of the oxidizer employed. The particle size of the powders canrange from about 10 to about 250 microns in diameter. Non-limitingexamples of metals that serve as burning catalysts are copper, vanadium,chromium, silver, molybdenum, sinconium, antimony, manganese, iron,cobalt, and nickel. Examples of metal oxide burning catalysts are ferricoxide, aluminum, copper oxide, chromic oxide, as well as the oxides ofthe other metals mentioned above.

The fuel particles that are employed in a solid propellant grain areusually a metal or a metal alloy, preferably the fuel contains 1 or moremetals of groups I-A, II-A, III-A and groups I-B through VII-B, groupIII of the Periodic Table. Thus, the metals may contain group I-Aelements such as lithium and group IIA metals such as beryllium ormagnesium. Illustrative of the group III-A metals is aluminum. Themetals of groups I-B through VII-B include copper, silver, zinc,manganese, iron, nickel, platinum and the like. Particularly preferredfor inclusion in the polymer matrix are aluminum, beryllium and lithiumsince these metals are of relatively low molecular weight giving lowmolecular weight combustion products in addition to having high heats ofcombustion.

Burning rate depressants and modifiers are also sometimes advantageouslyadded to the solid propellant grain of this invention. These aregenerally compounds which tend to inhibit burning reaction rates orabsorb heat and include specifically carbonyl chloride, oximide,nitroguanidine, guanidine nitrate, and oxalic acid.

As previously described, the improved properties obtained by themodification of a telechelic polymer are particularly applicable in thefield of solid propellant compositions where the improved properties areextremely beneficial. The following examples illustrate the manufactureof solid propellants utilizing the modified binders of this invention,and the improved properties obtained therefrom.

Example 11 A propellant composition utilizing a modified telechelicpolybutadiene polymer was manufactured. Into a two and one-halfBaker-Perkins propellant mixer was carefully weighed 426.48 grams (17.77weight percent) of modified telechelic carboxy-terminated polybutadiene.The polybutadiene was modified with vinylcyclohexene as described inExample I. Additionally weighed into the mixer was 384 grams (16.00weight percent) of finelypowdered aluminum metal which acts as the fuel.The mixer was then turned on to disperse the aluminum in the liquidbinder material. During this process a vacuum was applied to the mixer.After the aluminum had been dispersed within the binder material, 1584grams (66.00 weight percent) of the oxidizer which was ammoniumperchlorate was weighed and added to the mixer which was then turned onfor a short period, approximately two or three minutes to disperse theoxidizer. The vacuum was then applied and mixing was continued until theconstituents therein reached the temperature of l60170 F. at which timethe mixing was stopped. The temperature is reached due to thecirculating of a heating fluid in the jacket that surrounds the mixer.After the mixing had stopped and the cure temperature had been reached,

5.52 grams (.23 weight percent) of a curative which was MAPO(tris[l-(2-methyl)-aziridinyl]phosphine oxide) was added to the mixedconstituents without any vacuum being applied. The mixer was then turnedon to disperse the curative within the mixture. After the curative hadbeen dispersed, the contents were then poured into a pan 8 by 8 inchesand 3 inches deep and cast at a cure temperature of 170 for 48 hours.The curing time can vary and often the mixture is cured at 96 hours at150 F., it being understood that the cure time decreases with increasein temperature. The resultant solid propellant having the modifiedcarboxy-terminated polybutadiene polymer was obtained. The propellantwas then stamped out in the JANAF dogbone configuration for use indetermining its physical properties.

Example Ill Grams Wt.

Percent Garb cry-terminated linearrpolybutadiene modified withl,3-bis[3(2,3epoxyproxy)propyl] tetra methyl disiloxaue 445 17. 8Ammonium perchlorate, 1, 650 66. 0 Aluminum 10. 0 MAPO 5 20 A propellantcomposition utilizing a modified polybutadiene binder was obtained.

Example IV To illustrate the improved properties obtained with the novelpolymeric binders of this invention, tests were run comparing thepropellant compositions prepared according to Examples II and III with acontrol propellant which had the following compositions:

Percent by weight Linear carboxy-terminated polybutadiene (unmodifiedteleohelic polymer) l8 Ammonium perchlorate 66 Aluminum 16 The resultsobtained are set forth in Table I.

TABLE I Test Temp Control Example I Example II Deg. F

em, percent 26 48 51 S..., p.s.i

so 88 89 E, .s.i 455 275 255 cm, percent 34 G2 G6 Sm, p.s.i- 124 E...,psi. 5 10 345 330 em, percent- 32 51 51 m. p.s.i- 247 241 235 E,..,p.s.i 2, 010 1, 660 2,020

In Table I, e is the elongation at maximum tensile strength, expressedin percent; S is the maximum tensile strength in p.s.i.; and E is thetangent modulus of elasticity. All test specimens were standard JANAFdogbones. All testing was done on an Instron machine at a strain rate of0.77 in.-in. min. Of considerable import is the fact that the propellantelongation increased from an average of about 30 percent for the controlpropellant over the entire range of temperatures to an average exceeding50 percent for the propellant utilizing the modified binder of thisinvention. As can readily be seen from the table, the percent elongationnearly doubled utilizing the concepts set forth herein. Additionally, ascan be seen from the table, the maximum tensile strength increasedexcept for a slight decrease at 70. This is a significant aspect of theeffect of the present invention, since the elasticity of the propellantcan now be greatly increased without any detrimental affect on themaximum tensile strength. A'sign'ificant advance of the art has beenaccomplished in providing a propellant binder that will provide superiorand improved physical properties over an extreme range of temperaturesas seen in Table I.

Although the invention has been described and illustrated in detail, itis to be clearly understood that the same is by way of illustration andexample only and is not to be taken by way of limitation, the spirit andscope of this invention being limited only by the terms of the appendedclaims.

I claim:

1. The compound obtained by the reaction of carboxyterminatedpolybutadiene and 1,3-bis [3(2,3-epoXy-propoxy)propyl] tetra methyldisiloxane.

References Cited by the Examiner UNITED STATES PATENTS 2,848,442 8/1958Svetlik 260-82.1 3,053,708 9/1962 Hall et al. 149-19 3,055,781 9/1962Yamamoto 14919 3,070,583 12/1962 Uraneck 26082.1 3,087,844 4/1963 Hudsonet al. 14919 3,147,161 9/1964 Abere et a1 14919 FOREIGN PATENTS 226,6344/ 1959 Australia.

LEON I. BERCOVITZ, Primary Examiner. REUBEN EPSTEIN, C. D. QUARFORTH,Examiners.

L. A. SEBASTIAN, B. R. PADGETT,

Assistant Examiners.

1. THE COMPOUND OBTAINED BY THE REACTION OF CARBOXYTERMINATEDPOLYBUTADIENE AND 1, 3-BIS (3, (2, 3-EXPOXY-PROPOXY) PROPYL) TETRAMETHYL DISILOXANE.
 3. THE METHOD OF IMPROVING THE PHYSICAL STRENGTHPROPERTIES OF CARBOXY-TERMINATED POLYBUTADIENE WHICH COMPRISES REACTINGSAID POLYBUTADIENE WITH 1, 3-BIS (3(2, 3EPOXY-PROPOXY) PROPYL) TETRAMETHYL DISILOXANE.